1. Technical Field
The present invention relates to signal processing, and more particularly to a low complexity precoder design for large scale multiple-input and multiple-output (MIMO) communication systems.
2. Description of the Related Art
Multiple antenna systems, a.k.a., multiple-input multiple-output (MIMO) systems, have been an important solution for improving system throughput in the new generation high-speed wireless and cellular systems. The MIMO technique has been adopted in 3GPP Long Term Evolution (LTE) systems, which are being currently deployed worldwide. To further improve MIMO channel gain, transmission with eight transmit antennas is supported in the LTE Advanced (LTE-A) standard (3GPP LTE Release 10) with up-to eight data transmission layers for single user (SU) MIMO. However, it is difficult to equip such a large number of antennas at the user terminal end, particularly, on smart-phones, which makes the realization of the large spatial multiplexing gain less attractive to the wireless vendors. On the other hand, multi-user MIMO with linear precoding has been included in the LTE-A standard to further enhance system capacity. By serving multiple users on the same channel resources, high order MIMO can be virtually realized even with one receive antenna for each user due to the uncorrelated channel among users.
Recently, MIMO systems employing a large number of transmit antennas, e.g., tens or more than one hundred, draw increasing interest. These systems are often referred to as large-scale MIMO or massive MIMO. It has been shown that when supporting multiuser MIMO transmission with the transmit antennas in a large scale, the intra-cell interference among co-scheduled users vanishes due to uncorrelated channel and precoding matrices. Consequently, the network capacity can be significantly improved. One condition here for interference suppression utilizing precoding with a large number of transmit antennas is perfect knowledge of the channel information at the base station for all served users, which can be assumed in time-division duplex (TDD) systems. When the number of transmit antennas is in a higher order than the number of users, the system capacity is then scaled by the number of users. Moreover, matched filter based precoding and a linear receiver can be sufficient in large-scale MIMO systems, resulting in significantly low complexity of the signal processing at both the transmitter and receiver. Due to the throughput and complexity advantages, large-scale MIMO becomes more and more attractive given the current situation of sparse available spectrum and exponentially increasing wireless data demands.
Although match filter based precoding can perform fine in a large-scale MIMO system, with the antenna power constraints, the performance can be degraded when the transmitter has to scale the precoded signals among all transmit antennas to avoid signal distortions caused by amplifier limitations.